Various geotextiles are employed in erosion control, turf reinforcement, and civil constructions involving earth reinforcement. Geotextiles employed in earth reinforcement of level and graded structures, e.g. roadways or runways, and foundations typically have more biaxial geotextile tensile and/or shear strength properties than those geotextiles employed in erosion control and turf reinforcement. In addition, geotextiles used in earth reinforcement applications have more symmetrical tensile and/or shear strength properties than earth reinforcement materials employed in retaining wall structures and steep grades. These more level, more biaxial, and less aggressive environments accordingly place a premium on geotextiles which perform acceptably from a subgrade stabilization and base course reinforcement point of view, but which can be manufactured and supplied efficiently and inexpensively, and which can be rolled, stored, shipped, and installed easily.
Subgrade stabilization is often required when weak subgrade conditions exist. For subgrade stabilization, a geotextile is generally placed directly on top of a weak subgrade. The geotextile provides separation between an aggregate base course above and the subgrade below; improves bearing capacity; enables, potentially, a reduction in base course thickness; allows increased traffic; and reduces permanent deformation within a surface or pavement system placed on top of base courses. Separation, reinforcement, and filtration properties are relevant when considering geotextiles for subgrade stabilization applications.
Separation geotextiles minimize aggregate penetration into the underlying subgrade by the action of applied loads and subsequent migration of the subgrade upwardly into the base course. For example, it is known that an intermixing of as little as 10 to 20 percent of subgrade fines into the base course can severely damage base course strength. By employing a separation geotextile, contamination of a granular and/or aggregate base course by subgrade fines is effectively reduced, thereby preventing strength damage. Moreover, the presence of the separation geotextile can result in the thickness of the base course being reduced from that which otherwise would be necessary in the absence of the geotextile.
In addition, the disposition of a geotextile over the subgrade can significantly reduce the potential mode of failure and improve bearing capacity. The geotextile aides in the prevention of the granular and/or aggregate base course from punching into the soft foundation soils under direct applied loads, such as from wheel or truck loads. Absent the protection of the geotextile, base punching, or localized shear failure, can result in a general shear failure. The geotextile provides the subgrade an opportunity to develop its ultimate bearing capacity.
Soil deformation is directly related to the presence of a weak subgrade. As deformation of the soil occurs, large scale tension develops in the geotextile. Accordingly, the geotextile should provide tensioned-membrane support. The stress conditions in the base course under load are analogous to a loaded beam. Due to bending, the base experiences compression at the top and tension at the base under the load. The cohesionless base course material has no tensile resistance and generally relies on the subgrade to provide lateral restraint. Weak subgrades provide very little lateral restraint; thus, the aggregate at the bottom of the base course tends to move apart, allowing intrusion of the soft subgrade. By positioning a geotextile at the bottom of the base course, the geotextile restrains aggregate movement by providing tensile strength. The net effect is a change in the magnitude of stress imposed on the subgrade, a reduction directly under the loaded area and an increase outside the loaded area. This spreading of the stresses over a larger area improves the load carrying capability of the civil structure (e.g. a road). A geotextile possessing a high modulus can provide more load spreading ability for the same rut depth. Reinforcement through tensioned-membrane support is, therefore, provided through the geotextile's load-strain characteristics and soil/geotextile frictional interaction.
Yet, water flow rate and soil retention are at odds with conventional fabric strength. Typically, to increase strength, the pores of the fabric are reduced. As a result, the fabric is limited to the amount of water that can pass through the fabric and, as a result, the size of the soil particulates it can retain. If higher flow rates and larger particle size retention are desired, the fabric must yield on strength due to lower fabric density. Accordingly, there is a need for a woven geosynthetic fabric which has improved strength for reinforcement while maintaining relatively high flow rates and particle retention. It is to solving this and other needs the present invention is directed.